1. Field of Invention
The present invention relates to a type of data transmission line. More particularly, the present invention relates to a type of data transmission line for connecting a microprocessor and a chipset.
2. Description of Related Art
In general, a microprocessor or a central processing unit (CPU) inside a personal computer is able to communicate with peripheral devices via a chipset. The chipset is an intermediate element for the exchange of data and control signals. The chipset has input/output leads that couple with a data transmission bus, and the bus leads to a connector above a main circuit board. Hence, any microprocessor plugged into the connector is able to communicate with the chipset directly.
Currently, the two most important bus specifications include gunning transceiver logic (GTL+) and high-speed transceiver logic (HSTL). GTL+ bus is a standard specification created by Intel for transmitting data between a new generation of their microprocessors and external interfaces. The GTL+ bus is suitable for high-speed microprocessors such as the Pentium II, Pentium III, the Pentium Pro and Socket 370. On the other hand, HSTL bus is an alternative specification employed by some microprocessors. The GTL+ bus and the HSTL bus are really two different types of specifications. Hence, one chipset has to be used to interface with a microprocessor that employs a GTL+ bus while another chipset has to be used to interface with a microprocessor that employs a HSTL bus.
FIG. 1 is a schematic diagram showing a GTL+ data bus linking a microprocessor with a chipset. FIG. 2 is a schematic diagram showing a HSTL bus linking another microprocessor with a chipset. A few similarities between the transmission buses shown in FIGS. 1 and 2 can be found. Terminal voltages V.sub.TT for both of them are identical, for example, V.sub.TT =1.5V. Reference voltages V.sub.REF for both of them are also identical at about 1.0V (if V.sub.TT =1.5V), or V.sub.REF = 2/3*V.sub.TT or 0.68*V.sub.TT. Both the GTL+ bus 12 and the HSTL bus 22 use the same type of connectors 14 and 24 having identical dimensions. A microprocessor 16 having its own printed circuit board 16a is shown in FIG. 1. The circuit board 16a is plugged into a connector 14 above a main circuit board 10a so that the microprocessor 16 is connected to a chipset 10. Similarly, a microprocessor 26 having its own printed circuit board 26a is shown in FIG. 2. The circuit board 26a is plugged into a connector 24 above a main board 20a so that the microprocessor 26 is connected to a chipset 20.
A comparison of the GTL+ bus and the HSTL bus shows that their differences lie mainly in the arrangement of the transmission lines. The GTL+ transmission line 12 in FIG. 1 has one or two 56 ohms pull-up resistors R.sub.tt to increase the bus voltage level. Because the resistor R.sub.tt also happens to be close to the end of the transmission line, the resistor serves also as an end-termination resistor capable of preventing signal ring back. On the other hand, the HSTL transmission line 22 in FIG. 2 has two 100 ohms pull-up resistors R.sub.tt to increase bus voltage level. The resistors R.sub.tt do not serve as an end-termination resistor. The HSTL transmission line 22 further includes a serial resistor R.sub.s of about 22 ohms between the chipset 20 and the input/output (IO) terminals of the microprocessor 26. The resistor R.sub.s mainly serves as a damper for transmission signals.
The aforementioned description illustrates that GTL+ bus and HSTL bus are configured to follow two specifications from two different types of microprocessors. As a result, different chipsets must be used. Since a chipset is usually fixed onto the main board by manufacturers, a user's choice of microprocessor is limited.